Introduction

Photovoltaics is the generation of electrical power from solar radiation. Solar energy belongs to the renewable (regenerative) energy sources. This means that it is a source of electricity generation that never runs dry. Other renewable energy sources are wind and water power, geothermal energy and biomass from renewable resources such as wood or straw.

Renewable energy resources are different from non-renewable (fossil) resources such as oil, coal or natural gas. These fossil resources do not renew themselves quickly after energy has been produced from them. That is why they are becoming increasingly rare and therefore more expensive. Another difference to renewable energy resources is that energy from fossil resources is harmful to the climate and the environment, e.g. because CO2 is released during combustion.

The use of renewable energy sources, such as solar energy, is, therefore, an alternative way of conserving natural resources on the one hand and generating climate-friendly electricity on the other. The following pages show how this actually works and how photovoltaics can be used in buildings.

When it comes to photovoltaics, you always come across certain terms and abbreviations.
W … Watt (unit for power)
kW … Kilowatt (= 1,000 Watt)
kWh … kilowatt hour (unit for energy)
kWh/m2a … kilowatt hours per square meter and year

• Electrons: electrically negatively charged particles that circle around the atomic nucleus on orbits and thus form the so-called atomic shell. When there is a surplus or shortage of electrons in a body (e.g. a metal plate) (e.g. due to friction or radiation), the body is charged negatively or positively. However, because charge strives for balance, the excess electrons move to where electrons are missing. So there is a movement among the electrons. The electricity that flows is exactly such a movement.
• Photons: The word is derived from the Greek: Phos means light. Roughly speaking, photons can be called “light particles”. They make up electromagnetic radiation (like light, but also, for example, X-rays).
• Semiconductors: solids (substances), which can be both electrical conductors and non-conductors. Whether or not they conduct depends on external conditions (e.g. temperature) or on internal properties. For example, certain foreign atoms can be introduced into a semiconductor to give it the desired conductivity. This is called “doping” and is important for the production of solar cells, as we will see later.
• Airmass (AM); refers to the length of the path taken by light rays through the atmosphere to the ground, compared with the shortest path, i.e. the path taken when light is incident vertically. An air mass of 1 corresponds to a vertical incidence of light (angle of 0°) when the sun is at its zenith. An air mass of 1.5 corresponds to a light incidence of about 48° when the light rays hit the earth’s surface at an angle.
• Primary energy: naturally occurring energy in its original form before it is converted into electricity or fuel, for example. Primary energy sources include coal, oil and gas deposits, uranium, hydroelectricity, solar radiation, wind power, geothermal energy and biomass. After conversion, it is referred to as secondary energy and finally as the final energy that reaches the consumer.
• Efficiency: describes the efficiency of energy conversion in photovoltaics. How much solar energy is converted into electrical energy by a solar cell? The efficiency is either designated η (Eta) or given in percent: For example, a solar cell has an efficiency of 16 %, or one says: η = 0.16.
• Parallel connection: The elements of a circuit are connected in parallel if their negative poles are connected on one side and their positive poles on the other side. The current does not flow as a whole through all the elements one after the other (this would be “series connection”, see below), but splits up beforehand. With a parallel connection, the total current is increased while the voltage remains the same.
• Series connection: The elements of a circuit are connected in series when the same current flows through them in series. This allows higher total voltages to be generated while the current remains the same. The series connection is also called a series connection. The opposite is the “parallel connection” (see above).